As cyclists, we love our metrics, and 2020 has brought a buzzy new one to the forefront: pulse oximetry. Apple launched the Apple Watch Series 6 on September 18, touting pulse oximetry (or blood oxygen saturation, measured as SpO2) as one of the new tools putting “the future of health...on your wrist.” The Fitbit Sense and Versa 3 followed just a week later, calling SpO2 levels a key trend in tracking your health and well-being. Garmin’s Fenix, Forerunner, and vivoactive watches have been tracking this data since 2018.
The more mainstream this feature gets, the more interested people—especially athletes—are in devouring the data to see how it might inform their training. But before you get caught up in your SpO2 numbers, you need to understand what exactly pulse oximetry is and how accurate these devices are.
What exactly is pulse oximetry?
“Pulse oximetry is a non-invasive way to measure the saturation of oxygen carried in your red blood cells,” explains Jonathan Parsons, M.D., a pulmonologist at The Ohio State University Wexner Medical Center. Pulse oximeters can be attached to your fingers, forehead, nose, foot, ears, or toes, but you’re probably most familiar with the finger sensor commonly used in doctor’s offices.
Now, smartwatches have redesigned their wrist-based optical heart sensor to add blood oxygen measurement capabilities. “The sensor emits infrared light, and when that light hits your blood cells, it’s absorbed differently by those that have hemoglobin with oxygen on them versus those that don’t,” says Parsons. “Then, it can quantify that difference and give a percentage of red blood cells that are actually carrying oxygen.”
Are the results from these smartwatches as accurate as what you’d get in a doctor’s office, hospital setting, or even from an at-home finger sensor? Probably not, says Parsons. “It’s challenging because the oxygen levels in the back of the wrist may or may not reflect what’s going on in the rest of the rest of the body,” explains Malhotra.
FWIW, Apple stipulates that its blood oxygen app is “not intended for medical use” and is “only designed for general fitness and wellness purposes;” Fitbit’s disclaimer says its blood-oxygen app is “not intended for medical purposes, nor is it intended to diagnose, treat, cure, or prevent any disease or condition;” and Garmin clarifies in its app that “Pulse Ox data is intended for recreational use only [and] should not be treated as any type of medical diagnosis or treatment of disease.”
But the fact that wrist-based oximeters can only provide an estimation of your SpO2 levels isn’t necessarily a bad thing. For data-driven cyclists, says Parsons, “they’re accurate enough to give you a ballpark number and frame of reference as to how your numbers are trending.”
Why do blood oxygen levels matter?
Understanding what’s normal for you in terms of different health metrics can be valuable, just in terms of helping you stay on top of what’s going on in your body. Healthy blood oxygen levels are important because oxygen is what fuels your cells, tissues, muscles, and organs.
“Normal SpO2 values are in the high or even mid- to low 90s,” says Atul Malhotra, M.D., a critical care pulmonologist and sleep medicine specialist at UC San Diego Health. “And in a normal, healthy person, those oxygen levels should stay very stable. Healthy people generally don’t need to track their blood oxygen levels, because your heart and lungs will react automatically to decreases in oxygen levels by increasing your heart rate or breath rate to maintain normal oxygen levels.
If your SpO2 levels are below normal values, that could be an indication of a potential underlying cardiopulmonary issue, says Malhotra, including different kinds of heart and lung disease. It could also be a sign of something more mild like asthma, a respiratory infection, pneumonia, or even COVID-19 (more on that in a minute), he adds.
Doctors use pulse oximeters on people who are experiencing shortness of breath or who have a lung or heart condition; measuring their SpO2 levels can determine if they’re getting enough oxygen or not. “For someone with chronic lung or heart problems, they can be valuable in staying on top of their illness,” says Parsons.
Slight decreases in SpO2 levels are common at night, but if you’re someone who wakes up feeling unrefreshed, and you see that your nighttime SpO2 levels are low, it could be a sign of undiagnosed sleep apnea, a potentially serious sleep disorder in which breathing repeatedly stops and starts, says Malhotra. In that case, you should see a sleep specialist.
What does any of this have to do with your training?
Honestly, not that much. For now, pulse oximetry readings are just another metric for data-obsessed athletes to dig into. However, there is one training situation where tracking your SpO2 levels can be informative.
From an exercise perspective, the real value of a pulse oximeter is if you’re going to be training [or racing] at altitude, says Parsons. At higher elevations, the air has a lower concentration of oxygen in it than it does at sea level, which means you’re not going to be getting as much oxygen to your bloodstream. At the same time, “your body reduces the volume of your bloodstream in order to improve the oxygen-carrying capacity of your red blood cells (especially in the first 24 to 48 hours at a higher altitude),” says Parsons. “And that could cause issues with performance.”
To clarify: Your SpO2 levels aren’t something you’d check mid-workout. In order to get a reading, you have to hold your wrist really still for 15 seconds to a minute, so you’re likely not going to do that unless it’s before or after a workout. But if you feel crummy, less efficient, more fatigued before or after a workout, or like it takes you longer to recover than it used to, those numbers can be a helpful reminder that your body is acclimating to that higher elevation (even if you’re not feeling it in terms of shortness of breath). FYI: Those feelings are all normal when adjusting to altitude, and it can take up to four weeks for you to feel normal.
In terms of performance, if you’re a cyclist who is just training for riding, your oxygen levels are probably not a great gauge of much of anything, says Malhotra.
A caveat about COVID-19...
Pulse oximetry has become an especially buzzy topic around COVID-19. Sales of finger pulse oximeters in the United States spiked by 527 percent the week the first case of COVID-19 was confirmed in the United States, spiked again in mid-February, and have been increasing ever since, according to data published by Quartz.
“COVID-19 is a virus that attacks the lungs, and obviously that can potentially affect your ability to provide oxygen to your red blood cells,” says Parsons. A pulse oximeter, in theory, could alert someone to a potential COVID-19 symptom.
However, “respiratory symptoms related to illness do not always correlate with your SpO2 levels,” says Parsons. “What that means is you can be pretty sick and feel pretty terrible, but your pulse oximetry reading is normal. And vice versa.”
With COVID-19, the catch is your oxygen levels probably won’t drop until the illness is further along. But, “if somebody thinks they have COVID-19—let’s say they have mild symptoms, a runny nose or they can’t smell or they have a cough—in that context, oxygen monitoring oxygen levels may be useful,” says Malhotra.
Still, a smartwatch with SpO2-level measurements is not a reliable way to avoid contracting coronavirus or to track COVID-19 symptoms. It’s better to stick to the advice of experts: wash your hands regularly, wear a mask, and social distance as much as possible. If you notice COVID-19 symptoms, stay home, seek testing in your area, and take a break from riding until you get a diagnosis as exercise and COVID-19 can be a dangerous combination.
Otherwise, tracking your vital signs and staying alert to any significant abnormalities is never a bad idea—and could be enough of a warning for you to seek medical care. Worst case scenario? They send you home with a clean bill of health.
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